1. Field of the Invention
This invention concerns a deformable diaphragm pressure sensor with integrated anti-shock protection means. The invention also relates to a gradiomanometer incorporating such a sensor.
2. The Related Art
The pressure sensor in accordance with the invention may be used in any situation in which the use of a sensor, and especially its handling, involves a high risk of shocks which can lead to the deformable diaphragm bursting. Applications involving this risk occur particularly in the oil industry, in which the handling effected at the wellhead in particular often results in the rods and tools which are inserted into the well being dropped. Thus a differential pressure sensor in accordance with the invention can be used in gradiomanometers serving to measure the density of the fluid present in oil boreholes or wells, and also in apparatus for measuring the speed of displacement of a fluid.
In order to effect pressure measurements in oil wells it is known to use pressure sensors in which the sensitive element is formed by a diaphragm adapted to deform when a very small pressure difference exists between the two sides of the diaphragm. The measurement of the deformation of the diaphragm is effected by means of a bridge of piezo-resistive strain gages associated with the diaphragm.
In French patent application No. 93 04228, a very sensitive pressure sensor is proposed, using a very thin silicon diaphragm (for example around 30 .mu.m), on which the bridge of strain gages is formed using technologies comparable with those used in microelectronics.
In such a pressure sensor, the deformable diaphragm closes in sealed manner a passage containing a liquid such as a silicone oil of known density which transmits in full the pressures obtaining at the two ends of the passage. The diaphragm is thus biased or loaded with a fluid column of a certain height, which represents an applied pressure on the diaphragm.
When the tool in which the pressure sensor is fitted suffers a shock, for example because the tool has fallen a few centimeters, the fluid column is accelerated, which results in the application of a peak pressure to the diaphragm for the duration of the shock (in the order of a few .mu.s to a few ms), with an amplitude depending on the height of the fall and on the duration of the shock. Thus a fall of 10 cm on to a solid floor (concrete for example) results in a pressure peak of 10 bars for some tens of milliseconds on a diaphragm loaded with a column of 0.5 m.
Having regard to the measurement range of the diaphragm (some tenths of a bar), it will be understood that in such situations the diaphragm is destroyed. In view of the handling conditions at wellheads, this fragility of existing pressure sensors to shock results in a considerable increase in the cost of measurements effected with the aid of such sensors, which cannot in general be used without being often irreversibly damaged.
U.S. Pat. No. 3 616 688 describes a gradiomanometer for an oil well. In order to detect the pressure difference between two points separated by a known vertical distance, equipment is used that is vertically movable inside a tubular body disposed in a well. That movable equipment is filled with liquid and comprises two deformable bellows connected together by a rigid tube. The vertical position of the movable equipment inside the tubular body is a function of the static pressure difference exerted by the fluid present in the well on the two bellows. The pressure differential is thus determined by measuring the position of the movable equipment in the tubular body. An expansion chamber bounded by a third bellows communicates through a restriction with the lower bellows, in order to absorb variation in the volume of liquid contained in the movable equipment, without affecting the dynamic operation of the movable equipment.
The device described in U.S. Pat. No. 3 616 688 is also sensitive to shocks. Thus, the considerable pressure which is applied to the lower bellows of the movable equipment risks permanent deformation of the bellows and error in the measurements subsequently effected. In that document it is proposed to overcome the problem by fitting a normally-closed valve in parallel with the restriction which connects the lower bellows of the movable equipment to the bellows of the expansion chamber, which valve opens automatically when a downwards acceleration peak is applied to the apparatus as a result of a shock.
Given the totally different natures of the pressure sensors, the solution proposed in U.S. Pat. No. 3 616 688 is clearly not applicable to a deformable diaphragm pressure sensor.